Strain sensor with fixing members

ABSTRACT

A strain sensor improved in detection accuracy without variation of bending stresses applied to the strain detecting element. The sensor substrate has a first fixing hole at one end and a second fixing hole at the other end, a detecting hole at the center, and at least one strain detecting element on the upper surface or lower surface thereof. The first fixing member is press-fitted into the first fixing hole. The second fixing member is press-fitted into the second fixing hole. The detecting member is press-fitted into the detecting hole. Further, when an external force is applied to the detecting member, strain generated due to the positional shift of the detecting member is detected by the strain detecting element with respect to the first fixing member and the second fixing member.

This application is a U.S. National Phase Application of PCTInternational Application PCT/JP02/09375.

TECHNICAL FIELD

The present invention relates to a strain sensor for detecting straingenerated in a sensor substrate due to external forces produced byweights of men and vehicles such as automobiles by means of straindetecting elements mounted on the sensor substrate.

BACKGROUND ART

As a conventional strain sensor of this type, the one disclosed in themicrofilm of Utility Model H4-265 (Japanese Laid-open Utility ModelH5-57605) is well-known.

A conventional strain sensor will be described in the following withreference to the drawings.

FIG. 13 is a perspective view of a conventional strain sensor.

In FIG. 13, sensor substrate 1 made of metal has first fixing hole 2 atone end and second fixing hole 3 at the other end, on which is disposedstrain detecting sensor 4 having a thin gauge for detecting strain in alengthwise direction.

Also, strain detecting element 4 is electrically connected to elementsection 5 and is also provided with lead 6 outwardly extended.

The operation of a conventional strain sensor configured as describedabove is explained with reference to the drawings.

As shown in FIG. 14, sensor substrate 1 is fixed by male screw 9 ontodetecting member 8 provided with a pair of female screws 7. In thiscase, taking into account the variation of pitch of the pair of femalescrews 7 formed in detecting member 8, the inner diameters of firstfixing hole 2 and second fixing hole 3 are a little larger than theouter diameter of female screw 7 so that sensor substrate 1 can bereliably fixed on detecting member 8.

In this condition, when an external force is applied to detecting member8, sensor substrate 1 is deformed in accordance with strain in detectingmember 8. As sensor substrate 1 is deformed, resistance value of straindetecting element 4 disposed on the top surface of sensor substrate 1varies. The variation of the resistance value is taken out from lead 6in the form of voltage variation when a current flows to detectingelement 4. In this way, the external force generated at detecting member8 is detected.

In the conventional configuration described above, the inner diametersof first fixing hole 2 and second fixing hole 3 are a little larger thanthe outer diameter of female screw 7. Therefore, it is possible toreliably install sensor substrate 1 onto detecting member 8. On theother hand, when an external force is applied to detecting member 8, theinstalling position of sensor substrate 1 shifts in the lengthwisedirection of sensor substrate 1. This causes the position of end portion10 of male screw 9 being in contact with the top surface of sensorsubstrate 1 to be shifted. Accordingly, the bending stress applied tostrain detecting element 4 is varied as well. As a result, the outputaccuracy of the strain sensor will be deteriorated.

SUMMARY OF THE INVENTION

The present invention is intended to provide a strain sensor which mayassure the improvement of detection output accuracy without variation ofthe bending stress applied to the strain detecting element.

The strain sensor of the present invention has a configuration asdescribed in the following.

The sensor substrate of the strain sensor is provided with a firstfixing hole at one end and a second fixing hole at the other end, adetection hole at the center, and at least one strain detecting elementat the top or bottom thereof. A first fixing member is press-fitted intothe first fixing hole. A second fixing member is press-fitted into thesecond fixing hole. A detecting member is press-fitted into a detectinghole. Further, when an external force is applied to the detectingmember, strain caused due to the positional shift of the detectingmember in relation to the first fixing member and the second fixingmember is detected by the strain detecting element.

In this configuration, the first fixing member, the second fixing memberand the detecting member are respectively press-fitted into the firstfixing hole, the second fixing hole and the detecting hole. Accordingly,when an external force is applied to the detecting member, therespective end positions of the detecting member, the first fixingmember, and the second fixing member which are in contact with thesensor substrate are free from positional shifting. Consequently, thebending stress applied to the strain detecting element is stabilized.Accordingly, the detection output accuracy of the strain sensor will beimproved.

Thus, it is possible to obtain a strain sensor which may assure theimprovement of detection accuracy without variation of the bendingstress applied to the strain detecting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a strain sensor in the preferred embodiment 1 ofthe present invention.

FIG. 2 is a side sectional view of a strain sensor in the preferredembodiment 1.

FIG. 3 is a perspective view of a first upper washer, essential portionof the strain sensor, in the preferred embodiment 1.

FIG. 4 is a perspective view of a first cylinder, essential portion ofthe strain sensor, in the preferred embodiment 1.

FIG. 5 is a top view of a sensor substrate, which is an essentialportion of the strain sensor, in the preferred embodiment 1.

FIG. 6 is a sectional view showing the structure of a locking means forlocking a first fixing member to the sensor substrate, which is anessential portion of the strain sensor, in the preferred embodiment 1.

FIG. 7 is a side sectional view showing an operational state of thestrain sensor in the preferred embodiment 1.

FIG. 8 is a top view of a strain sensor in the preferred embodiment 2 ofthe present invention.

FIG. 9 is a side sectional view of a strain sensor in the preferredembodiment 2.

FIG. 10 is a perspective view of a first upper washer, which is anessential portion of the strain sensor, in the preferred embodiment 2.

FIG. 11 is a top view of a sensor substrate, which is an essentialportion of the strain sensor, in the preferred embodiment 2.

FIG. 12 is a sectional view showing the structure of a locking means forlocking a first fixing member to the sensor substrate, which is anessential portion of the strain sensor, in the preferred embodiment 2.

FIG. 13 is a perspective view of a conventional strain sensor.

FIG. 14 is a side sectional view showing a state that the strain sensoris fixed to a detecting member, which is an essential portion in theperspective view of the conventional strain sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Preferred Embodiment 1)

The preferred embodiment 1 of the present invention will be described inthe following with reference to FIG. 1 to FIG. 7.

FIG. 1 is a top view of a strain sensor in the preferred embodiment 1 ofthe present invention.

FIG. 2 is a side sectional view of the strain sensor.

FIG. 3 is a perspective view of a first upper washer in the strainsensor.

FIG. 4 is a perspective view of a first cylinder in the strain sensor.

FIG. 5 is a top view of a sensor substrate in the strain sensor.

FIG. 6 is a sectional view showing the structure of a locking means forlocking a first fixing member to a sensor substrate of the strainsensor.

As shown in FIG. 5, sensor substrate 11 is provided with first fixinghole 12 ranging from top to bottom at one end, second fixing hole 13ranging from top to bottom at the other end, and detecting hole 14ranging from top to bottom at the center thereof.

Also, first fixing hole 12, second fixing hole 13, and detecting hole 14of sensor substrate 11 are respectively provided with recesses 11 a atthe inner sides thereof.

Also, power electrode 15 is provided at the other end of sensorsubstrate 11. Power electrode 15 is electrically connected to one end offirst strain detecting element 16 and one end of second strain detectingelement 17 via circuit pattern 18.

In this case, the other end of first strain detecting element 16 iselectrically connected to first output electrode 19.

Further, the other end of second strain detecting element 17 iselectrically connected to one end of second output electrode 20 and oneend of third strain detecting element 21. The other end of third straindetecting element 21 is electrically connected to ground (GND) electrode22.

Also, fourth strain detecting element 23 is disposed on the top surfaceof sensor substrate 11. One end of fourth strain detecting element 23 iselectrically connected to the other end of first strain detectingelement 16 and to first output electrode 19. The other end of fourthstrain detecting element 23 is electrically connected to GND electrode22.

First strain detecting element 16, second strain detecting element 17,third strain detecting element 21, fourth strain detecting element 23,power electrode 15, first output electrode 19, second output electrode20, GND electrode 22 and circuit pattern 18 configure a bridge circuit.

Also, IC 24 is disposed on the top surface at the other side of sensorsubstrate 11. The IC 24 amplifies the differential voltage between thevoltages of first output electrode 19 and second output electrode 20 andto deliver the output from external output electrode 25 disposed at theother end of sensor substrate 11.

Further, external power electrode 26 is disposed on the top surface atthe other side of sensor substrate 11. External power electrode 26 iselectrically connected to power electrode 15.

Also, external GND electrode 27 disposed on the top surface of sensorsubstrate 11 is electrically connected to GND electrode 22.

First fixing member 28 shown in FIG. 1 and FIG. 2 includes first upperwasher 29, first lower washer 30, and first cylinder 31.

In this case, first upper washer 29 is formed of metal having a shape asshown in FIG. 3, and the underside thereof comes in contact with the topsurface of sensor substrate 11.

First lower washer 30 is configured in the same way as that for firstupper washer 29, and the top surface thereof comes in contact with theunderside of sensor substrate 11.

First cylinder 31 is formed of thin metal as shown in FIG. 4.

First cylinder 31 is press-fitted into the inner sides of first upperwasher 29, first lower washer 30, and first fixing hole 12.

Also, first upper washer 29 and first lower washer 30 are respectivelyprovided with recesses 11 a at the inner sides thereof.

As shown in FIG. 6, the outer side of first cylinder 31 partially getsinto these recesses 11 a and recesses 11 a of first fixing hole 12,thereby locking first fixing member 28 to sensor substrate 11.

In this way, as shown in FIG. 6, locking means 11 b for first fixingmember 28 is configured.

Second fixing member 32, in the same manner as that for the first fixingmember 28, includes second upper washer 33 made of metal, second lowerwasher 34, and second cylinder 35 formed of thin metal.

Second cylinder 35 is press-fitted into the inner sides of second upperwasher 33, second lower washer 34, and second fixing hole 13. Further,second upper washer 33 and second lower washer 34 are respectivelyprovided with recesses 11 a at the inner sides thereof. The outer sideof second cylinder 35 partially gets into these recesses 11 a andrecesses 11 a of second fixing hole 13, thereby locking second fixingmember 32 to sensor substrate 11. In this way, locking means 11 b forsecond fixing member 32 is configured.

Detecting member 36 includes detector upper washer 37 made of metal,detector lower washer 38, and detecting cylinder 39 formed of thinmetal.

Detecting cylinder 39 is press-fitted into the inner sides of detectorupper washer 37, detector lower washer 38, and detecting hole 14.Detector upper washer 37 and detector lower washer 38 are respectivelyprovided with recesses 11 a at the inner sides thereof. The outer sideof detecting cylinder 39 partially gets into these recesses 11 a andrecesses 11 a of detecting hole 14, thereby locking detecting member 36to sensor substrate 11. In this way, locking means 11 b for detectingmember 36 is configured.

In the strain sensor of the preferred embodiment 1 of the presentinvention, when an external force is applied to detecting member 36, theexternal force is applied to end portion 36 a where detecting member 36comes in contact with sensor substrate 11. Then, a force (reactionforce) in the opposite direction of the external force is applied to endportion 28 a where first fixing member 28 comes in contact with thesensor substrate 11. Further, a reaction force is also applied to endportion 32 a where second fixing member 32 comes in contact with sensorsubstrate 11.

The strain sensor is configured as described above. Accordingly, endportion 36 a where detecting member 36 comes in contact with sensorsubstrate 11, end portion 28 a where first fixing member 28 comes incontact with sensor substrate 11, and end portion 32 a where secondfixing member 32 comes in contact with sensor substrate 11 are free frompositional shifting.

As a result, bending stresses applied to first strain detecting element16, second strain detecting element 17, third strain detecting element21, and fourth strain detecting element 23 are stabilized. Accordingly,the output accuracy of the strain sensor will be improved.

Also, first fixing member 28, second fixing member 32, and detectingmember 36 are provided with locking means 11 b. Therefore, first fixingmember 28, second fixing member 32, and detecting member 36 do notrotate about the axial centers of first fixing hole 12, second fixinghole 13, and detecting hole 14 respectively.

Consequently, end portion 36 a of detecting member 36, end portion 28 aof first fixing member 28, and end portion 36 a of second fixing member36, coming in contact with sensor substrate 11, are free from positionalshifting.

Accordingly, bending stresses applied to first strain detecting element16, second strain detecting element 17, third strain detecting element21, and fourth strain detecting element 23 are further stabilized. Thus,the output accuracy of the sensor will be further improved.

The assembling method of the strain sensor in the preferred embodiment 1of the present invention which is configured as described above isexplained in the following.

First, first fixing hole 12, second fixing hole 13, and detecting hole14 are formed by press machining in a metallic base material (notshown). Also, recesses 11 a are formed by press machining at therespective inner sides thereof. In this case, press machining causesburrs to be produced at positions around first fixing hole 12, secondfixing hole 13 and detecting hole 14 of the base substrate (not shown).

Next, after glass paste (not shown) is printed on the top surface of thebase material (not shown), the base material is burnt at about 850° C.for about 45 minutes to form sensor substrate 11.

Subsequently, metal-glazed carbon paste is printed on the top surface ofsensor substrate 11, followed by burning at about 850° C. for about 45minutes. Thus, first strain detecting element 16, second straindetecting element 17, third strain detecting element 21, and fourthstrain detecting element 23 are formed on top surface of the sensorsubstrate 11.

Next, silver paste is printed at a position where an electrode andcircuit pattern 18 are disposed, followed by burning at about 850° C.for about 45 minutes. Thus, power electrode 15, first output electrode19, second output electrode 20, GND electrode 22, external outputelectrode 25, external output electrode 26, external GND electrode 27,and circuit pattern 18 are formed.

Next, IC24 is mounted on the top surface of sensor substrate 11.

Next, after first lower washer 30 is in contact with the lower side offirst fixing hole 12 of sensor substrate 11, first upper washer 29 isplaced on upper side of first fixing hole 12.

Subsequently, first cylinder 31 is press-fitted into the inner sides offirst fixing hole 12, first lower washer 30 and first upper washer 29.In this case, the outer side of first cylinder 31 is locked in recesses11 a provided at the inner sides of first upper washer 29, first lowerwasher 30 and first fixing hole 12.

Next, after second lower washer 34 is in contact with the lower side ofsecond fixing hole 13, second upper washer 33 is placed on the upperside of second fixing hole 13.

Next second cylinder 35 is press-fitted onto the inner surfaces ofsecond fixing hole 13, second lower washer 34, and second upper washer33. In this case, the outer side of second cylinder 35 is locked inrecesses 11 a provided at the inner sides of second upper washer 33,second lower washer 34 and second fixing hole 13.

Subsequently, after detector lower washer 38 is in contact with thelower side of detecting hole 14 of sensor substrate 11, detector upperwasher 37 is placed at the upper side of detecting hole 14.

Finally, detecting cylinder 39 is press-fitted into the inner sides ofdetecting hole 14, detector lower washer 38 and detector upper washer37. In this case, the outer side of detecting cylinder 39 is locked inrecesses 11 a provided at the inner sides of detector upper washer 37,detector lower washer 38 and detecting hole 14.

In this configuration, thin first cylinder 31 is shrunk and press-fittedinto the inner sides of first upper washer 29, first lower washer 30 andfirst fixing hole 12. Also, second thin cylinder 35 is shrunk andpress-fitted into the inner sides of second upper washer 33, secondlower washer 34 and second fixing hole 13. Further, detecting thincylinder 39 is shrunk and press-fitted into the inner sides of detectorupper washer 37, detector lower washer 38 and detecting hole 14.

The above press-fitting operation may be easily performed. Accordingly,it is possible to provide a strain sensor improved in convenience forassembling.

In addition, first upper washer 29, first lower washer 30, and firstfixing hole 12 are provided with recess 11 a. Recess 11 a is locked bythe outer side of first cylinder 31.

Further, second upper washer 33, second lower washer 34, and secondfixing hole 13 are also provided with recess 11 a. Recess 11 a locks theouter side of second cylinder 35.

Further, detector upper washer 37, detector lower washer 38, anddetecting hole 14 are provided with recess 11 a. Recess 11 a locks theouter side of detecting cylinder 39.

Due to such a configuration, first fixing member 28, second fixingmember 32 and detecting member 36 can be easily provided with lockingmeans 11 b.

As a result, it is possible to easily provide a strain sensor havinglocking means 11 b.

The operation of the strain sensor in the preferred embodiment 1 of thepresent invention which is configured and manufactured as describedabove will be described in the following with reference to the drawings.

FIG. 7 is a side sectional view showing the operational state of thestrain sensor in the preferred embodiment 1 of the present invention.

As shown in FIG. 7, when external force F is applied to detecting member36 from the upper side, external force F causes sensor substrate 11 tobe deformed.

Then, an external force is applied to end portion 36 a of detectingmember 36 which is in contact with sensor substrate 11. Simultaneously,a force (reaction force) in the opposite direction of the external forceis applied to the end portion 28 a of first fixing member 28 which is incontact with sensor substrate 11. Also, a reaction force is also appliedto end portion 32 a of second fixing member 32 which is contact withsensor substrate 11.

In this case, tensile stresses in the lengthwise direction of sensorsubstrate 11 are applied to first strain detecting element 16 and thirdstrain detecting element 21 disposed on the top surface of sensorsubstrate 11. Due to the tensile stresses, the resistance values offirst strain detecting element 16 and third strain detecting element 21become increased in the lengthwise direction of sensor substrate 11.

At the same time, compressive stresses in the lengthwise direction ofsensor substrate 11 are applied to second strain detecting element 17and fourth strain detecting element 23. The resistance values of secondstrain detecting element 17 and fourth strain detecting element 23become decreased in the lengthwise direction of sensor substrate 11.

Further, first strain detecting element 16, second strain detectingelement 17, third strain detecting element 21 and fourth straindetecting element 23 are connected to each other by circuit patterns,thereby configuring a bridge circuit. And, the potential differencebetween first output electrode 19 and second output electrode 20 isdifferentially amplified and outputted by IC 24.

The external force F applied to detecting member 36 is detected fromexternal output electrode 25 according to the output voltage of IC 24.

Incidentally, locking means 11 b is preferable to be configured asdescribed in the following.

At the inner sides of first upper washer 29, first lower washer 30 andfirst fixing hole 12 of first fixing member 28 are respectively providedwith protrusions (not shown) instead of recesses. The outer side offirst cylinder 31 is locked by the protrusions (not shown).

Further, at the inner sides of second upper washer 33, second lowerwasher 34 and the second fixing hole 13 of the second fixing member 32are respectively provided with protrusions (not shown) instead ofrecesses. The outer side of the second cylinder 35 is locked by theprotrusions (not shown).

Further, at the inner sides of the detector upper washer 37, thedetector lower washer 38 and detecting hole 14 of detecting member 36are respectively provided with protrusions (not shown) instead ofrecesses. The outer side of detecting cylinder 39 is locked by theprotrusions (not shown).

Even in the case of configuring the locking means (not shown) asdescribed above, the same function as in the configuration describedearlier can be obtained by the preferred embodiment 1.

(Preferred Embodiment 2)

The preferred embodiment 2 will be described in the following withreference to FIG. 8 to FIG. 12.

FIG. 8 is a top view of a strain sensor in the preferred embodiment 2 ofthe present invention.

FIG. 9 is a side sectional view of the strain sensor.

FIG. 10 is a perspective view of a first upper washer of the strainsensor.

FIG. 11 is a top view of a sensor substrate of the strain sensor.

FIG. 12 is a sectional view showing the structure of a locking means forlocking a first fixing member to the sensor substrate of the strainsensor.

Those having the same configurations as in the preferred embodiment 1 ofthe present invention are given the same reference numerals, and thedescription is omitted.

As shown in FIG. 8 to FIG. 12, first fixing member 41, second fixingmember 45 and detecting member 50 of the strain sensor in the preferredembodiment 2 are configured as described in the following.

First fixing member 41 includes first upper washer 42 and first lowerwasher 43.

First upper washer 42 is press-fitted into first fixing hole 12 andcomes in contact with the top surface of sensor substrate 11. Firstlower washer 43 is press-fitted into first fixing hole 12 and comes incontact with the underside of sensor substrate 11.

As shown in FIG. 10, first upper washer 42 has protrusion 44. Also,first lower washer 43 has protrusion 44 as well.

These protrusions 44 are fitted in recesses 46 provided in first fixinghole 12 of sensor substrate 11 shown in FIG. 11. Thus, as shown in FIG.12, locking means 47 of first fixing member 41 is configured.

Further, relief groove 42 b is provided at the inner side of contactportion 42 a at which first upper washer 42 and first lower washer 43comes in contact with sensor substrate 11.

Also, second fixing member 45 includes second upper washer 48 and secondlower washer 49.

Second upper washer 48 is press-fitted into second fixing hole 13 andcomes in contact with the top surface of sensor substrate 11. Secondlower washer 49 is press-fitted into second fixing hole 13 and comes incontact with the underside of sensor substrate 11.

Further, second upper washer 48 and second lower washer 49 arerespectively provided with protrusions 44. These protrusions 44 arefitted in recesses 46 of second fixing hole 13 of sensor substrate 11.

Locking means 47 of second fixing member 45 is configured as describedabove.

Further, relief groove 42 b is provided at the inner side of contactportion 42 a at which second upper washer 48 and second lower washer 49comes into contact with sensor substrate 11.

Also, detecting member 50 includes detector upper washer 51 and detectorlower washer 52.

Detector upper washer 51 is press-fitted into detecting hole 14 andcomes in contact with the top surface of sensor substrate 11. Detectorlower washer 52 is press-fitted into detecting hole 14 and comes incontact with the underside of sensor substrate 11.

Further, detector upper washer 51 and detector lower washer 52 arerespectively provided with protrusions 44. These protrusions 44 arefitted in recesses 46 of detecting hole 14 of sensor substrate 11.

Locking means 47 of detecting member 50 is configured as describedabove.

Further, relief grooves 42 b is provided at the inner sides of contactportions 42 a which detector upper washer 51 and detector lower washer52 respectively come in contact with sensor substrate 11.

As described above, in the preferred embodiment 2 of the presentinvention, first fixing member 41, second fixing member 45 and detectingmember 50 are press-fitted in sensor substrate 11. Simultaneously formedare end portion 41 a, end portion 45 a and end portion 50 a at whichfirst fixing member 41, second fixing member 45 and detecting member 50respectively come in contact with sensor substrate 11.

Accordingly, it is possible to provide a strain sensor reduced in thenumber of parts and man-hour for assembling.

Also, according to the method as described earlier, locking means 47 isconfigured for first fixing member 41, second fixing member 45 anddetecting member 50. That is, locking means 47 can be formed only bypress-fitting first fixing member 41, second fixing member 45 anddetecting member 50 into sensor substrate 11.

Accordingly, it is possible to provide a strain sensor reduced inman-hour for assembling.

Here, as described in the preferred embodiment 1, when first fixing hole12, second fixing hole 13 and detecting hole 14 are formed by pressmachining in a metallic base material (not shown) used as sensorsubstrate 11, burrs may be generated around first fixing member 12,second fixing hole 13 and detecting hole 14 of the base material (notshown).

In the strain sensor in the preferred embodiment 2 of the presentinvention, relief grooves 42 b is provided at the inner sides of contactportions 42 a at which first upper washer 42, first lower washer 43,second upper washer 48, second lower washer 49, detector upper washer 51and detector lower washer 52 respectively come in contact with sensorsubstrate 11.

Accordingly, when first upper washer 42, first lower washer 43, secondupper washer 48, second lower washer 49, detector upper washer 51 andthe detector lower washer 52 are press-fitted in the sensor substrate11, burrs generated on sensor substrate 11 will get into relief grooves42 b. Consequently, first upper washer 42, first lower washer 43, secondupper washer 48, second lower washer 49, detector upper washer 51 anddetector lower washer 52 will not come loose against sensor substrate 11but may come in tight contact with sensor substrate 11.

Thus, end portion 50 a of detecting member 50, end portion 41 a of firstfixing member 41, and end portion 45 a of second fixing member 45 arestabilized with respect to their positions. Accordingly, it is possibleto further improve the output accuracy of the strain sensor.

Also, locking means 47 may be configured as described in the following.

Recesses (not shown) provided in place of protrusions engage first upperwasher 42 and first lower washer 43, and protrusions (not shown)provided in place of recesses engage the sensor substrate.

Further, recesses (not shown) provided in place of protrusions engagesecond upper washer 48 and second lower washer 49, and protrusions (notshown) provided in place of recesses engage sensor substrate 11.

Further, recesses (not shown) provided in place of protrusions engagedetector upper washer 51 and detector lower washer 52, and protrusions(not shown) provided in place of recesses engage sensor substrate 11.

Even in the case of configuring the locking means (not shown) asdescribed above, the same function as in the configuration describedearlier can be obtained by the preferred embodiment 2. Also, it ispossible to provide a strain sensor reduced in man-hour for assembling.

INDUSTRIAL APPLICABILITY

In the strain sensor of the present invention, when an external force isapplied to the end portion where the detecting member press-fitted intothe detecting hole of the sensor substrate comes in contact with thesensor substrate, a force (reaction force) in the opposite direction tothe external force is applied to the end portions where the first fixingmember and the second fixing member respectively come in contact withthe sensor substrate. In this case, the first fixing member ispress-fitted into the first fixing hole of the sensor substrate. Thesecond fixing member is press-fitted into the second fixing hole of thesensor substrate. In this configuration, when an external force isapplied, the end portions where the detecting member, the first fixingmember and the second fixing member come in contact with the sensorsubstrate are free from positional shifting. Accordingly, the bendingstress applied to the strain detecting element is stabilized. As aresult, the output accuracy of the strain detecting element isstabilized, and it is possible to provide a strain sensor improved inoutput accuracy.

1. A strain sensor, comprising: a sensor substrate provided with a first fixing hole and a second fixing hole, a detecting hole between the first fixing hole and the second fixing hole, and at least one strain detecting element; a first fixing member press-fitted into the first fixing hole; a second fixing member press-fitted into the second fixing hole; and a detecting member press-fitted into the detecting hole, wherein the first fixing member includes a first upper washer coming in contact with an upper surface of said sensor substrate and a first lower washer coming in contact with a lower surface of said sensor substrate, the second fixing member includes a second upper washer coming in contact with an upper surface of said sensor substrate, and a second lower washer coming in contact with a lower surface of said sensor substrate, wherein strain of the sensor substrate due to a positional shift of said detecting member in relation to the first fixing member and the second fixing member is detected by the strain detecting element when an external force is applied to said detecting member.
 2. The strain sensor of claim 1, wherein each of the first fixing member, the second fixing member and the detecting member has a locking means.
 3. The strain sensor of claim 1, wherein the first fixing member further includes: a first thin cylinder which is press-fitted into inner surfaces of the first upper washer and the first lower washer; and the second fixing member further includes: a second thin cylinder which is press-fitted into inner surfaces of the second upper washer and the second lower washer; and the detecting member includes: a detector upper washer coming in contact with an upper surface of said sensor substrate, a detector lower washer coming in contact with a lower surface of said sensor substrate, and a detecting thin cylinder which is press-fitted into inner side surfaces of the detector upper washer and the detector lower washer.
 4. The strain sensor of claim 3, wherein recesses or protrusions for locking an outer side surface of the first cylinder are provided at the inner side surfaces of the first upper washer, the first lower washer, and the first fixing hole, recesses or protrusions for locking an outer side of the second cylinder are provided at the inner side surfaces of the second upper washer, the second lower washer, and the second fixing hole, and recesses or protrusions for locking an outer side surface of the detecting cylinder are provided at the inner side surfaces of the detector upper washer, the detector lower washer, and the detecting hole.
 5. The strain sensor of claim 1, wherein in the first fixing member, the first upper washer is press-fitted into the first fixing hole, and the first lower washer is press-fitted into the first fixing hole, and in the second fixing member, the second upper washer is press-fitted into the second fixing hole, and the second lower washer is press-fitted into the second fixing hole, and said detecting member includes: a detector upper washer which is press-fitted into the detecting hole and comes in contact with an upper surface of said sensor substrate, and a detector lower washer which is press-fitted into the detecting hole and comes in contact with a lower surface of said sensor substrate.
 6. The strain sensor of claim 5, wherein the first upper washer and the first lower washer are respectively provided with recesses or protrusions, and the first fixing hole is provided with protrusions or recesses; the recesses of the first upper washer and the first lower washer engage the projections of the first fixing hole, or the protrusions of the first upper washer and the first lower washer engage the recesses of the first fixing hole; the second upper washer and the second lower washer are respectively provided with recesses or protrusions, and the second fixing hole is provided with protrusions or recesses; the recesses of the second upper washer and the second lower washer engage the projections of the second fixing hole, or the protrusions of the second upper washer and the second lower washer engage the recesses of the second fixing hole; the detector upper washer and the detector lower washer are respectively provided with recesses or protrusions, and the detecting hole is provided with protrusions or recesses; and the recesses of the detector upper washer and the detector lower washer engage the projections of the detecting hole, or the protrusions of the detector upper washer and the detector lower washer engage the recesses of the detecting hole.
 7. The strain sensor of claim 5, wherein the first upper washer, the first lower washer, the second upper washer, the second lower washer, the detector upper washer, and the detector lower washer is provided with a relief groove at an inner side of an contact portion thereof coming in contact with said sensor substrate. 